Aluminium metal is presently produced by electrolysis of an aluminium containing compound dissolved in a molten electrolyte, and the electrowinning process is performed in smelting cells of conventional Hall-Hèroult design. These electrolysis cells are equipped with horizontally aligned electrodes, where the electrically conductive anodes and cathodes of today's cells are made from carbon materials. The electrolyte is based on a mixture of sodium fluoride and aluminium fluoride, with additions of alkaline and alkaline earth halides. The electrowinning process takes place as the current passed through the electrolyte from the anode to the cathode causes the electrical discharge of aluminium ions at the cathode, producing aluminium metal, and the formation of carbon dioxide on the anode (see Haupin and Kvande, 2000). The net reaction of the process can be illustrated by the equation:2Al2O3+3C=4Al+3CO2  (1)
Due to the horizontal electrode configuration, preferred electrolyte composition and the use of consumable carbon anodes, the currently used Hall-Hèroult process display several shortcomings and weaknesses. The horizontal electrode configuration renders necessary an area intensive design of the cell and resulting in a low aluminium production rate relative to the footprint of the cell. The low productivity to area ratio results in high investment cost for green field primary aluminium plants.
Numerous attempts have been made to improve the currently used Hall-Hèroult process for production of aluminium metal. The improvements are aimed at cell design as well as electrode materials. One possible solution is the introduction of so-called inert electrodes, i.e. wettable cathodes (U.S. Pat. Nos. 3,400,036, 3,930,967 and 5,667,664) and oxygen evolving anodes (U.S. Pat. Nos. 4,392,925, 4,396,481, 4,450,061, 5,203,971, 5,279,715 and 5,938,914 and UK. Pat. No. 2 076 021 A). All of these patents are aimed at reducing the energy consumption during aluminium metal electrolysis through the implementation of so-called aluminium wettable cathode materials, as well as the removal of green house gasses from the electrolytic production of aluminium by applying oxygen-evolving anodes.
These “new” electrodes can be applied to both novel cell designs as well as in retrofitting of existing Hall-Hèroult cells. Patents regarding retrofit or enhanced development of Hall-Hèroult type of aluminium electrowinning cells are amongst others described in U.S. Pat. Nos. 4,504,366, 4,596,637, 4,614,569, 4,737,247, 5,019,225, 5,279,715, 5,286,359 and 5,415,742, as well as UK Pat. NO. 2 076 021 A. The major problem of the cell design suggested in these patents is, however, that the requirement for a large aluminium pool on the cell floor to provide electrical contact for the cathodes. This will render the cell susceptible to the influence of the magnetic fields created by the bus bar system, and may hence cause local short-circuiting of the electrodes when operating at short interpolar distances.
Novel cell designs for aluminium electrowinning are among others described in U.S. Pat. Nos. 4,681,671, 5,006,209, 5,725,744 and 5,938,914. Also U.S. Pat. Nos. 3,666,654, 4,179,345, 5,015,343, 5,660,710 and 5,953,394 describes possible designs of light metal electrolysis cells, although one or more of these patents are oriented towards magnesium production. Most of these cell concepts are applicable to multi-monopolar and bipolar electrodes.
Other Publications:
    Haupin, W. and Kvande, H.: “Thermodynamics of electrochemical reduction of alumina”, Light Metals 2000, pp. 379-384, 2000.    Lorentsen, O- A.: “Behaviour of nickel, iron and copper by application of inert cathodes in aluminium production”, Dr. Ing. thesis 2000/104, Norwegian University of Science and Technology, Trondheim, Norway, 2000.    Lorentsen, O- A. and Thonstad, J.: “Laboratory cell design considerations and behaviour of inert cathodes in cryolite-alumina melts”, 11th International Aluminium Symposium, Slovak—Norwegian Symposium on Aluminium Electrowinning, September 19-22, Norway, pp. 145-154, 2001.    McMinn, C., Crottaz, O., Bello, V., Nguyen, T. and deNora, V.: “The development of a metallic anode and wettable cathode coating and their tests in a 20-kA prototype drained cell”, Light Metals, 2002.    Solheim. A.: “Formation of solid deposits at the liquid cathode in Hall-Hèroult cell”, International Aluminium Symposium, Slovak—Norwegian Symposium on Aluminium Electrowinning, September 19-22, Norway, pp. 97-104, 2001.    Solheim. A.: “Crystallization of cryolite and/or alumina nay lake place at the cathode during normal cell operation”, Light Metals 2002, pp. 3 225-230, 2002Operating Oxygen Evolving, Inert Anodes:
With inert anodes in the electrowinning of aluminium oxide, the net reaction would be:2Al2O3=2Al+3O2  (2)
So far, no commercial scale electrolysis cells have been operated successfully over longer periods of time with inert anodes. Many attempts have been made to find the optimum inert anode material and the introduction of these materials in electrolytic cells. Proposed materials for inert anodes in aluminium electrolysis includes metals, oxide-based ceramics as well as cermets based on a combination of metals and oxide ceramics. The proposed oxide-containing inert anodes may be based on one or more metal oxides, wherein the oxides may have different functions, as for instance chemical “inertness” towards cryolite-based melts and high electrical conductivity (ex. U.S. Pat. Nos. 4,620,905 and 6,019,878). The proposed differential behaviour of the oxides in the harsh environment of the electrolysis cell is, however, questionable (see McMinn et al. (2002)).1. The metal phase in the cermet anodes may likewise be a single metal or a combination of several metals. The main problem with all of the suggested anode materials is their chemical resistance to the highly corrosive environment due to the evolution of pure oxygen gas (1 bar) and the cryolite-based electrolyte. To reduce the problems of anode dissolution into the electrolyte, additions of anode material components to saturate the electrolyte with anode components (U.S. Pat. No. 4,504,369) and a self generating/repairing mixture of cerium based oxy-fluoride compounds (U.S. Pat. Nos. 4,614,569, 4,680,049 and 4,683,037) have been suggested as possible inhibitors of the electrochemical corrosion of the inert anodes. However, none of these systems have been demonstrated as viable solutions.
When operating cells with inert anodes, one major and often prohibitive problem is the accumulation of anode material elements in the produced aluminium metal due to the electrochemically assisted dissolution of the anode material in the electrolyte. Several patents have tried to address this problems by suggesting a reduction in the cathode surface (U.S. Pat. Nos. 4,392,925 and 4,681,671), i.e. the surface of the produced aluminium metal. Reduced aluminium metal surface exposed to electrolytic bath will reduce the uptake of dissolved anode material components in the metal, and hence increase the durability of the oxide-ceramic (or metals and cermets) anodes in the electrolysis cells. This is amongst others described in U.S. Pat. Nos. 4,392,925, 4,396,481, 4,450,061, 5,203,971, 5,279,715 and 5,938,914 and in UK. Pat. No. 2 076 021 A.
During electrolysis of aluminium metal, heat is generated in the process. In the traditional Hall-Hèroult cells, as well as in any novel design cells, heat will be generated due to the electrical resistance of the current bearing components of the cell. The major heat generating materials/components will be the anode and the electrolyte. The heat generation in the anode is dependent on the electrical conductivity of the anode materials, and the heat generation in the electrolyte will depend on the electrolyte composition and the distance between the anode and the cathode ion the cell, i.e. the interpolar distance (ACD). It is a well known fact that most materials/anode components will have a decreased solubility in molten cryolite based electrolyse as the temperature of the bath decreases. Hence, another and yet more feasible route to suppress metal contamination, would be to reduce the dissolution of the anode components in the electrolyte by reducing the anode temperature and or the electrolyte temperature. As presented in patent number WO 01/31090, the most recent inert anode materials may consist of mixtures of NiO and FeO with metallic additions of Cu, in which some Cu metal may be oxidised during sintering and/or electrolytic operation to form CuO. As indicated in FIG. 1, based on data collected from Lorentsen (2000), it is obvious that the major inert anode material components will exhibit a decreased solubility as the temperature decrease. By arranging the electrodes and cell design in order to keep the anodes as the coldest part of the cell interior, the dissolution rates into the bath will be reduced. If the anode is mainatined at a temperature slightly lower than the electrolyte, there will be a thermal impetus for depositing dissolved anode material on the anode itself rather than on the surrounding structure elements of the cell, i.e. the dissolution of anode material components will be suppressed.
U.S. Pat. No. 4,737,247 propose the use of heat pipes embedded in the anode current conductor rod (anode stem). The main purpose of the heat pipes in the sited patent is to protect some of the structural elements of the inert anode assembly, i.e. the spacer, from chemical erosion by molten electrolyte, by assuring the formation of a protective layer of frozen bath around these structural elements. The heat pipes are, however, not designed to keep the anode surface colder than the electrolyte, and as such reduce the dissolution of anode material in the electrolyte.
Operating Aluminium Wetted Cathodes:
Inert, or wettable cathodes are usually proposed manufactured from so-called Refractory Hard Materials (RHM) like borides, nitrides and carbides of the transition metals, and also RHM silicides are proposed as useful as inert cathodes (U.S. Pat. Nos. 4,349,427, 4,376,690 and 2001/0020590). The RHM cathodes are readily wetted by aluminium metal and hence a thin film of aluminium metal may be maintained on the cathode surfaces during aluminium electrowinning in drained cathode configurations. This wetting of the cathodes is the key to successful operation of the wetted cathodes, especially if the cathodes are employed in a vertical or tilted/sloped design geometry. Under these circumstances it is essential that the produced aluminium metal is drained off the cathode and not allowed to accumulate in the interpolar space and thus enabling the cell or parts of the cell to short circuit.
Solheim (2001) addressed the problem of formation of solid deposits at the cathode during electrolysis. Solids depositions at the cathode during electrolysis is caused by precipitation and adherence of bath components, often infiltrated with a metal phase. When aluminium electrolysis takes place, aluminium is formed at the cathode surface. Because of the migration of sodium ions, as current carriers, also towards the cathode, the cryolite ratio of the bath at the cathode surface (i.e. catholyte) will decrease compared to the bulk electrolyte (Solheim, 2001), as illustrated in FIG. 2. As a result of this change of bath composition, the liquidus temperature of the catholyte will be different from the liquidus temperature of the bulk bath, and hence under given conditions solid deposits of cryolite and/or alumina may form at the cathode, as is illustrated in FIG. 3. This has been confirmed experimentally in a laboratory scale cell with inert electrodes, as reported by Lorentsen (2000) and is shown in FIG. 4. The rate of formation of the solid deposits is dependent on, amongst others, bath composition (cryolite ratio), bath temperature, superheat, alumina concentration and cathodic current densities.
The formation of solid deposits on the cathode may grow once formed and percolate the continuous aluminium film on the drained cathodes, hence accounting for electrical passivation of the cathode are as well as promoting the growth of large aluminium balls on the cathode surface. Due to the lack of or reduced wetting of aluminium on the cathode surface caused by the solid deposits, the aluminium balls (spheres) will continue to grow under cathodic polarisation and may eventually short circuit the cell or parts of the cell when reaching the adjacent cathode surface.